1. Field of the Invention
This invention relates to a method of separating L-cysteine from a solution prepared by the reaction of L-serine. More particularly, it is a method of separating L-cysteine as the monohydrate of its hydrochloride.
L-Cysteine is an unstable compound which is easily oxidized. Therefore, it is usually sold in the form of its hydrochloride. L-Cysteine and its hydrochlorides are mainly used as medicines or materials for medicines, or additives to foods or cosmetics. The use thereof for preparing a cold-wave solution has recently shown a particularly great increase. L-Cysteine is an amino acid containing sulfur.
2. Description of the Prior Art
There are known a variety of methods for producing L-cysteine. They include (1) a method relying upon extraction from a natural substance, (2) a method relying upon organic synthesis, (3) a method relying upon fermentation, and (4) a method using an enzyme. The method relying upon extraction has, however, the disadvantages that there is only an unstable supply of raw materials, and that the extracted product is likely to contain other unnecessary amino acids. The method relying upon fermentation has the disadvantage of being lower in productivity than the method using an enzyme. Therefore, the method using an enzyme is considered to be more advantageous than any other method from the standpoint of industrial application.
There are known a number of methods which synthesize L-cysteine by employing enzymes. They include (1) a method employing cysteine synthetase to synthesize L-cysteine from L-serine and hydrogen sulfide, and (2) a method employing serine sulfhydrase to synthesize L-cysteine from L-serine and hydrogen sulfide, or from .beta.-chloroalanine and hydrogen sulfide. Japanese Patent Application No. 84545/1985 discloses the method which was invented by the inventors of this invention. It employs sulfhydryl synthetase for reacting L-serine and a metal hydrosulfide to produce L-cysteine.
Whichever method may be employed, however, it is very difficult to separate L-cysteine from a solution obtained as a product of reaction, since the solution has a complicated composition, and since L-cysteine is highly soluble in water.
When the method using an enzyme or relying upon fermentation is employed, a large amount of an inorganic salt, such as sodium chloride, is formed when the pH of the solution is controlled during its reaction, or when bacterial impurities are removed from the aqueous reaction product. The product unavoidably contains cystine as a by-product of the reaction. Moreover, it contains the unreacted L-serine which need be collected therefrom, as it is expensive. While it is necessary to refine the reaction product to separate L-cysteine from any such inorganic salt, unreacted L-serine and cystine, it is difficult to separate L-cysteine effectively without causing any appreciable loss thereof, since it has a high degree of solubility in water.
Therefore, there is known a method which oxidizes L-cysteine in the reaction product into L-cystine, which is less soluble in water, instead of separating it directly. The L-cystine thereby formed is separated from the reaction product and converted again to L-cysteine by electrolytic reduction, etc. It is, however, evident that this method is entirely unstable for industrial application from the standpoints of yield, operation and cost.